The invention relates to a deflection unit for a cathode ray tube, the deflection unit comprising line deflection coils, frame deflection coils surrounding the line deflection coils, and a yoke ring having a magnetic permeability xcexcr and surrounding the frame deflection coils.
The invention also relates to a cathode ray tube provided with a deflection unit, and to a display apparatus comprising such a cathode ray tube.
A deflection unit comprising frame deflection coils and line deflection coils is known from WO-A 98/26574. A deflection unit of this type is generally constructed in such a way that the two coils are mounted on a hollow support (one on the inner side and one on the outer side) and that a yoke ring surrounds these coils. The purpose of the yoke ring is to short-circuit the magnetic lines of flux outside the coils, which are generated by the frame and line coils during operation. The yoke ring thus reduces unwanted external magnetic stray fields. The yoke ring also reflects the stray fields into a deflection volume, i.e. the volume within the cathode ray tube in which the deflection of electron beams takes place. This also leads to a reduction of the power that is dissipated in the deflection coils. However, a further reduction is desired.
It is an object of the invention to provide a deflection unit for a cathode ray tube wherein the dissipated power for the deflection is reduced. To this end, the deflection unit according to the invention is characterized in that the deflection unit comprises a magnetic material which is present between the line deflection coils and the frame deflection coils and has a magnetic permeability xcexc1 which satisfies the relation xcexc1 less than xcexcr.
In the deflection unit according to the invention, the dissipation of the energy is reduced because the magnetic field generated by the line coils is reflected by the magnetic material into the deflection volume. However, a proper selection of the value of xcexc1 is important. The inventors have realized that, only if xcexc1 less than xcexcr, the magnetic field generated by the frame coils will be transmitted towards the deflection region, and a reduction of dissipated energy can be obtained. For the frame coils, the function of the magnetic material depends on the value of the magnetic permeability xcexc1 of the magnetic material within the deflection unit. If the permeability in this region is comparable to or higher than that of the yoke ring xcexcr, the magnetic material acts as a reflector. Hence, the magnetic field generated by the frame coils is reflected away from the deflection volume. This deteriorates the performance of the frame coils.
However, if the condition xcexc1 less than xcexcr is satisfied, the magnetic material will act as a flux guide, which will lead to an enhancement of the magnetic field of the frame coil inside the deflection volume. Consequently, the current through the flame coils may be reduced, leading to a reduction of ohmic losses in the frame coils as well as a reduction of dissipation in the electronic drive circuits.
It is to be noted that WO-A 00/44028 describes a deflection unit in which void spaces between wire strands within the frame deflection coils and/or void spaces between the yoke ring and the frame coils are filled with magnetic material. No limitation with respect to the relative permeability of the magnetic material is required in this case.
Advantageous embodiments of the invention are defined in the dependent claims.
These and other aspects of the invention will be elucidated with reference to the embodiments described hereinafter.